An inner tank of a domestic hot water heater is usually formed by a cylindrical steel container having a circumferential side wall, a top dome shell and a bottom wall shell which are welded together. The container is provided with holes to receive fittings which are welded or screwed thereto. These fittings are required to secure piping to the tank, electric heating elements if the hot water heater is to be heated by electricity, a sacrificial anode and other devise such as temperature sensors, etc. The sacrificial anode, or sacrificial rod, is a metallic anode used in cathodic protection where it is intended to be dissolved to protect other metallic components inside the tank. The more active metal is more easily oxidized than the protected metal and corrodes first, hence the term “sacrificial”, and it generally oxidizes nearly completely before the less active metal will corrode, thus acting as a barrier against corrosion for the protected metal. Therefor, the more metal that is exposed inside the tank wall the faster the sacrificial anode will deteriorate and any exposed metal surface will start oxidizing.
Oxidization of exposed metals in hot water heater has been found problematic in the past and attempts have been made to construct the inner casing of the hot water heater of non-corrosive materials. U.S. Pat. No. 5,379,507 describes such a method of manufacture wherein an inner shell of non-corrosive material is cast inside the outer shell while heat is applied to the outer shell. The leak-tight liner is a polymer material such as polyethylene, polyprolyn or nylon. An objective of this design was to substantially reduce the amount of steel required to fabricate the tank as well as preventing corrosion. This would also eliminate the need of a sacrificial anode thereby resulting in a further cost reduction. However, such tanks have not proven efficient due to other problems that it created.
We have found that the life expectancy of the inner steel tanks of hot water heaters can be improved greatly if defects in the glass lining on the inner wall can be eliminated or substantially reduced. One of the major problems which cause corrosion is due to sharp transition areas inside the tank wall and which are caused by weld couplings, element couplings and primarily the joint formed inside the tank wall by the top dome shell and the cylindrical steel shell.